The electronic phototypesetting system as herein described uses an output ray tube to reproduce light emissive characters for exposure of photosensitive film. Prior art systems require complex character generation apparatus to provide electrical signals representative of character images. Basically two types of character generating systems have evolved, namely, digital systems which require large memories for storing the necessary data to generate the electronic control signals which are necessary to reproduce the character; and "analog" systems which use some form of signal generating cathode ray tube (CRT) assembly which is coupled to and responsive to the outlines of characters in a group of characters selected to be reproduced. It is common to arrange such a selection in a grid pattern or matrix referred to herein as a font wherein each character is positioned within, and in relation to, the spatial bounds of an elemental area of the matrix. There are a number of well-known basic signal generating CRT arrangements which could be employed as a character generator. They include the flying spot scanner, the monoscope type tube and a variety of light pickup (camera) tubes such as the image orthicon, the vidicon, the image dissector and variations thereof. These above arrangements have seen only limited application in the prior art due to technical limitations and complexities including attainable resolution, elimination of geometric and signal distortions, maintenance of stability and provision of the control flexibilities necessary to generate an adequate range of output characters and formatting.
As a result such prior art CRT systems have been limited substantially to low quality (non-graphic arts) application; or they have employed various arrangements of optical, mechanical, electronic elements in hybrid systems; or they have employed a multiplicity of loosely coupled CRT's, each generating a limited few characters; or they have employed a single tube with a multiplicity of output signals. An example of the latter is the CBS/Mergenthaler "Linotron" employing a highly special image dissector tube having a multiplicity (256) of signal channels -- one for each character. These prior art systems are therefore overly complex, and expensive and/or limited in performance, reliability and flexibility.
It has been possible to overcome the basic above difficulties, first by recent developments resulting in certain substantially higher resolution camera tubes and second by development for this invention of deflection-focus components also capable of the high resolution and low distortion required to make a feasible system using the above noted basic arrangement employing a single character generator tube and having a single output signal. There are a wide variety of alternative equipment features, circuit arrangements, control arrangements, performance features, etc. which may be employed to provide a photocomposing machine of the above basic description. Many of these features and arrangements present mutual incompatabilities and/or lead to added complexities. As a result such a machine has not heretofore been devised.
The present invention describes an arrangement which overcomes these problems to result in a simple, low cost machine having low distortion, high reliability, high stability and incorporating a number of features to provide an arrangement of most desirable modes of operation as will now be described.
As previously noted a number of signal generating tubes could be used as the character generator. The system will be described using an image dissector tube (IDT) as a preferred embodiment. The basic IDT was invented by Farnsworth and used in the first early development of electronic television in 1923. It has much more recently undergone a number of performance improvements which make it possible to achieve its inherent high resolution capability. The tube in conjunction with equally high performance deflection and focusing components developed for this invention removes the limitation to utilization of the basic single CG tube -- single signal path character generator. The IDT responds essentially instantaneously to a wide range of lighting conditions without lag or delay characteristics exhibited by other signal pickup devices and is simple and reliable in operation.
A principal means provided to assure high accuracy and long term performance is horizontal and vertical automatic positioning circuitry to locate character scanning waveforms precisely in register with the area of each selected character at the start and continuously through generation of each character. This circuitry operates directly on the scan waveforms instead of requiring complex switching to alternative modes of operation for this function as in prior art. It is made compatible with character generation control requirements below. This feature is described in U.S. Pat. No. 3,497,761.
Electronic Typesetting has a basic advantage in that it is possible to generate, from a single font, characters which may be altered or modified in a number of ways. The character may be changed over a wide range in size by means of suitable point size control. Further, using set size control the output may be expanded or compressed. It may also be slanted or italicized, emboldened and also be altered in a variety of presently unconventional ways. Formatting is also subject to a wide range of control. Simplified methods for such control features are disclosed herein including a preferred selection of scanning sequence and waveforms.
A further desirable feature involves the reproduction of characters at a constant beam velocity and beam intensity independent of point/set size control adjustments. This results in a constant output rate of film exposure and eliminates variable control of beam intensity vs point/set size. Here again the feature is attained by preferred selection of the scanning sequence and waveforms and automatic control thereof.
In proportionally spaced typesetting each character has a relative width and the machine must be controlled accordingly to provide correct inter-character spacing. The character font when combined with a very high resolution pickup tube may be considered as a very large yet simple read-only-memory. This memory provides an ideal place to store coded character width information and means are devised to make such information retrieval and use occur compatibly and simultaneously with other functions of the machine. Means for use of width codes for line composition during film pulldown and for character spacing control during automatic positioning intervals are provided.